The check valves are for common aquarium air pumps. Please DO NOT use them with CO2 system. Great one way check valve for aquarium and fish tank. Protect you air pump by preventing aquarium water from back siphoning during a power outage. Made of durable clear plastic. Net weight: 0.14 oz / 4 g. Please use a paperclip to poke it from Input side to Output side when the outlet inside gets stuck. Air Hole Diameter: 0.15" / 4mm; Overall Size: 2.1" x 0.58" / 5.3 x 1.5 cm (L*Max.D) Package Content: 6 Pcs x Air Check Valve 6 Pack Red One Way Valves The nonreturn air pump check valves are made of durable plastic materials and designed to fit most common aquarium air pump while making no noise. Keep your air pump from getting water running into your pump when doing water changes or when the power is interrupted. Perfect for aquariums, fish tanks and gardening etc. Note: Please don't place the air valves in CO2 system because the high pressure will make them crack and leak. Fit standard 3/16 inch airline hoses Good air flow & no restrictions Made of rigid hard plastic Durable and aging resistant Long run time without noise How to Install 1. Before use, clean and dry both sides of the tubing so as to hook up valve tightly. 2. Check the flow direction of the valve to make sure put on the correct sides. 3. Attach air pump check valve to the tubing carefully and connect completely. Tips: If the valve restricts air flow in both directions, please poke something into the hole on the inlet side to push open the valve. After that the valve would allow airflow in the right direction.